The research aims of this study are to investigate the nature of the relationship that might exist between the sex chromosome aneuploidies in males (i.e, XXY, XYY) and subsequent personality development. Infants with these chromosome lesions--and infants serving as controls--were located through a chromosome screening of male neonates. These infants and children are being followed in a long-term developmental study with the aim of identifying reactive styles or developmental lags that might be related to the chromosome variations. The data, obtained through semistructural parental interviews (recorded verbatim) and directed observations of the child, are reduced to 16 operationally-defined categories of behavior that represent the child's development and functioning in the most relevant aspects of psychosocial adaptation. Considerable controversy has surrounded that issue of the possible relationship that might exist between certain sex chromosome variations and personality development. This controversy has resulted from the fact that much of the evidence suggesting such a relationship has been obtained by surveying groups of individuals who demonstrated a particular personality deviation and find an increasing incidence of a specific chromosome aneuploidy among them. Such studies--based on biased ascertainment methods--have had particular significance for individuals with the XYY karyotype, since these data have resulted in global assumptions and an extensive literature emphasizing the relationship that allegedly exists between this chromosome complement and "criminality." In actuality, the natural history of unselected individuals who carry such variations remains largely unknown. Unbiased information on the relation between the specific chromosome aberration and subsequent development can best be obtained from systematic study of aneuploid individuals who have not been preselected for any particular behavioral impairment or variation. Should long-term logitudinal studies of affected individuals reveal any developmental risk associated with the variation, this knowledge can be directed to helping these individuals overcome developmental lags and realize their full development potentials.